A typical circuit board assembly includes a circuit board and circuit board components mounted to the circuit board. In general, the circuit board includes multiple layers of conductive and non-conductive material sandwiched together into a rigid structure that physically supports the circuit board components, as well as provides a network of electrical pathways (i.e., signal traces, power supply signal planes, etc.) that (i) delivers power to the circuit board components and (ii) enables the components to communicate with each other. The circuit board components typically include, among other things, integrated circuit (IC) devices, individual electronic components (e.g., resistors, capacitors, diodes, etc.), and connecting devices (e.g., connectors, optical transducers, etc.).
Although some circuit board components are configured to mount to circuit boards in a press-fit manner (e.g., pins of the component package engage plated-through holes of the circuit boards), many components are configured to mount to circuit boards using solder. One common process for soldering circuit board components to a circuit board utilizes surface mount technology (SMT).
In one conventional SMT process, circuit boards pass horizontally through a series of automated equipment stations. For example, at a first station, automated equipment lowers a stencil on top of a circuit board. The stencil defines apertures that match pads on the surface of the circuit board. Accordingly, when the stencil is properly aligned with the circuit board, the stencil precisely exposes the circuit board pads and covers other circuit board areas that do not have pads. The automated equipment then spreads solder paste directly onto the tops of the circuit board pads (e.g., surface mounted component land patterns) by placing the solder paste on the stencil, and then wiping a squeegee across the stencil to push the solder paste through the apertures and onto the pads. The automated equipment subsequently removes the stencil thus leaving tiny erect bricks of solder paste covering the circuit board pads of the circuit board. As the circuit board moves to the next automated station, the printed solder paste tends to hold its shape and tends to resist slumping due to its thick and binding consistency.
At the next automated equipment station, automated pick and place equipment positions circuit board components on the circuit board. In particular, the automated equipment places SMT contacts of each component in contact with the printed solder paste which is further in direct contact with the circuit board pads.
Next, the circuit board passes through an oven that heats the circuit board, the solder paste, and the circuit board components. As the temperature increases, flux within the paste activates in order to clean both the circuit board pads and the contacts (or metallic interconnects/leads) of the circuit board components, i.e., the flux flows over and chemically reacts with the outer metallic surfaces in order to remove oxidized metal that would otherwise prevent the solder from properly wetting to the circuit board pads and the component contacts. Additionally, solder within the paste melts and reflows. That is, the solder enters the liquid state and wets to both the cleaned top surfaces of the circuit board pads (e.g., a cleaned nickel-gold plating on top of bare copper) as well as the SMT contacts of the circuit board components, e.g., solder balls of a Ball Grid Array (BGA) package, gull wing leads, metal contacts of a ceramic device, etc. Accordingly, after the circuit board exits the oven and cools off, robust and reliable solder joints exist between the circuit board components and the circuit board to rigidly secure the components to the circuit boards. At this point, a reliable electrical pathway exists between each component contact and a corresponding circuit board pad.
Some circuit board designs require components to be mounted to both sides of the circuit board. For such a design, the circuit board is turned over and the opposite side of the circuit board is processed by automated equipment in a similar manner as that described above for the first side of the circuit board.
Finally, a washing station washes the circuit board to remove contaminants. For example, in one type of washing station, automated equipment sprays cleaning solution onto both sides of the circuit board to carry away dirt, debris, flux resides, etc. from the circuit board surfaces. The resulting finished circuit board assembly is now ready for testing and operation.
It should be understood that there are a variety of solders which are presently available to circuit board manufacturers. For example, one type of solder is lead-based. That is, the solder is an alloy that includes a substantial amount of lead (e.g., more than 5%) among other types of metal (e.g., tin, silver, etc.). Manufacturers have found such lead-based solders to be particularly well-suited for forming robust and reliable solder joints between circuit board components and circuit boards. In particular, such lead-based solders tend to wet easily to the surfaces of circuit board pads, e.g., to copper or to nickel-gold plating which often coats the circuit board pads, even in the presence of only a relatively mild flux. Moreover, many lead-based solders are available in a “no-clean” formulation that does not require the manufacturer to subsequently clean (e.g., wash) the circuit boards after the soldering process, thus shortening manufacturing time and reducing manufacturing costs.
Some conventional circuit board manufacturing approaches apply lead-based solder to copper circuit board pads using a process called a Hot Air Solder Leveler (HASL) process prior to circuit board assembly. The HASL process involves vertically immersing a circuit board into a flux bath, subsequently vertically immersing the circuit board into a molten lead-based solder bath, and then air knifing away excess molten lead-based solder to leave behind an intermetallic boundary layer over the pad. Due to the varying geometries of the copper circuit board pads (i.e., different sized pads and pitches, pads in different directions, etc.), lumps of lead-based solder and solder shorts would remain on the circuit board without performing the air knifing procedure. The knife air pressure is set to create a flat fine pitch pad and scavenges off the excess lead-based solder leaving copper pads with a thin but strong intermetallic boundary layer. This remaining intermetallic boundary layer is typically within the range of 150 to 250 millionths of an inch thick. The part of the remaining boundary layer that is closest to the copper circuit board pads includes a relative high amount of copper. The middle of the remaining boundary layer includes both copper and the lead-based solder. The top most part of the remaining boundary layer includes mostly lead-based solder but still has copper which existed to hold that lead-based solder in place, i.e., otherwise that lead-based solder would have been blown away by the air knifing procedure.
Another type of solder is lead-free solder. Lead-free solder contains, at most, only trace amounts of lead (e.g., less than 1%) among other alloys (e.g., tin, silver, etc.). Lead-free solders are generally perceived as being more environmentally friendly than lead-based solders since circuit boards manufactured with lead-free solder have one less hazardous element (i.e., lead) that could later contaminate the environment. Lead-free solders are typically not available in a “no-clean” form and thus require cleaning after the soldering process in order to remove contaminants from the surfaces of the circuit board. Without such cleaning, the contaminants may lower the surface insulation resistance (SIR) of the circuit board. In some instances, failure to remove the contaminants may create a “wet battery” effect (i.e., ion migration) between metallic structures of the circuit board (e.g., adjacent pads or traces) eventually resulting in a short between the metallic structures.